Optical fiber cable

ABSTRACT

An optical fiber cable has a sectional area of Ac [mm 2 ] and housing a number N of optical fibers. A transmission loss α dB  [dB/km], a mode field diameter W [μm], an effective area Aeff [μm 2 ], an effective length L eff  [km], and a wavelength dispersion D [ps/nm/km] of each of the optical fibers at a wavelength of 1550 nm satisfy a predetermined equation and the transmission loss of the optical fiber at the wavelength of 1550 nm is 0.19 dB/km or less, and the effective area of the optical fiber is in a range from 125 to 155 μm 2 .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.15/084,784, filed on Mar. 30, 2016, for Optical Fiber Cable, whichclaims priority under 35 U.S.C. §119 to Japanese Patent Application No.2015-075141, filed on Apr. 1, 2015. The contents of these applicationsare incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to an optical fiber cable.

Description of the Related Art

With an optical communication system using an optical fiber arranged ina transmission line, it is desirable to transmit information in highvolume. In general, since an optical wavelength band used for an opticalcommunication system is limited to C band: 1530 to 1565 nm and L band:1565 to 1610 nm, to transmit information in high volume, it is desirablethat spectral efficiency (SE) [b/s/Hz] expressing a transmissioncapacity per frequency is high. Also, an optical fiber cable whichincludes and integrally covers a plurality of optical fibers is arrangedin, for example, a conduit line placed underground. Since the space inthe conduit line is limited, it is desirable to transmit information inhigher volume with an optical fiber cable having a smaller sectionalarea. Spectral efficiency per unit sectional area of an optical fibercable is expressed as a spatial spectral efficiency (SSE) [b/s/Hz/mm²].

Japanese Unexamined Patent Application Publication No. 2014-067020discloses an optical fiber that improves an optical signal to noiseratio (OSNR) while taking account of high-density packing in an opticalcable. Also, International Publication No. 2013/129234 discloses anoptical fiber that increases SE per unit sectional area of an opticalfiber. With the optical fiber described in Japanese Unexamined PatentApplication Publication No. 2014-067020, an effective area Aeff islimited to 100 μm² or smaller. Also, SSE of the optical fiber cable isnot considered. The optical fiber described in international PublicationNo. 2013/129234 is a multi-core optical fiber, but is not a single corefiber. Also, an optical fiber cable is not considered.

SUMMARY OF THE INVENTION

An object of the invention is to provide an optical fiber cable thatincreases SE per unit sectional area of the optical fiber cable.

An optical fiber cable according to an aspect of the invention has asectional area of Ac [mm²] and including a number N of optical fibers. Atransmission loss α_(dB)[dB/km], a mode field diameter (MFD) W [μm], aneffective area Aeff [μm²], an effective length L_(eff) [km], and awavelength dispersion D [ps/nm/km] of each of the optical fibers at awavelength of 1550 nm satisfy Eq. (1):

$\begin{matrix}{\log_{2}\left\lbrack {{{1 + {{65.9 \cdot \left\{ {{\exp\left( {200 \cdot \left( \frac{\alpha_{dB} + 0.02}{4.343} \right)} \right)} \cdot \left( \frac{20 \times W}{W^{2} + 104} \right)^{- 4} \cdot \left. \quad\left. {A_{eff}^{- 2}L_{eff}} \middle| D \middle| {}_{- 1}{\cdot {{asinh}\left( {629 \cdot} \middle| D \middle| L_{eff} \right)}} \right. \right\}^{- \frac{1}{3}}} \right\rbrack} \times \frac{N}{A_{C}}}} \geq {{0.008 \times N} + 1.7}},} \right.} & (1)\end{matrix}$wherein the transmission loss of the optical fiber at the wavelength of1550 nm is 0.19 dB/km or less, and the Aeff of the optical fiber may bein a range from 125 to 155 μm².

In the optical fiber cable according to the aspect of the invention, themode field diameter (MFD) of the optical fiber at the wavelength of 1550nm may be in a range from 12.0 to 13.5 μm. The wavelength dispersion ofthe optical fiber at the wavelength of 1550 nm may be in a range from 19to 22 ps/nm/km. The optical fiber may have a cutoff wavelength in arange from 1400 to 1600 nm.

The optical fiber included in the optical fiber cable according to theaspect of the invention may include a core and a cladding, a relativerefractive index difference

$\frac{n_{core} - n_{cladding}}{n_{core}}$of the core with respect to the cladding may be in a range from 0.28% to0.35%, and the core may have a diameter in a range from 12 to 15 μm.Alternatively, the optical fiber may include a core, an inner cladding,and an outer cladding, the outer cladding may have a refractive indexthat is smaller than a refractive index of the core and larger than arefractive index of the inner cladding, a relative refractive indexdifference

$\frac{n_{{outer}\mspace{14mu}{cladding}} - n_{{inner}\mspace{20mu}{cladding}}}{n_{{outer}\mspace{14mu}{cladding}}}$of the outer cladding with respect to the inner cladding may be in arange from 0.05% to 0.10%, a relative refractive index difference

$\frac{n_{core} - n_{{inner}\mspace{14mu}{cladding}}}{n_{core}}$of the core with respect to the inner cladding may be in a range from0.28% to 0.35%, and the core may have a diameter in a range from 12 to15 μm. A relative refractive index difference

$\frac{n_{core} - n_{{pure}\mspace{14mu}{silica}}}{n_{{pure}\mspace{14mu}{silica}}}$of the core with respect to pure silica may be in a range from −0.1% to+0.1%.

The optical fiber cable according to the aspect of the invention may bea ribbon slotted-core cable, and a value obtained by dividing asectional area of a single slot by a number of the optical fibers housedin the single slot may be in a range from 0.12 to 0.16 mm²/core.

With the aspect of the invention, the optical fiber cable that increasesSE per unit sectional area of the optical fiber cable can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an optical fiber cable according toan embodiment of the invention.

FIG. 2 is a cross-sectional view of an optical fiber cable according toan embodiment of the invention.

FIG. 3 is a cross-sectional view of an optical fiber cable according toan embodiment of the invention.

FIG. 4 is a cross-sectional vi an example of a slot included in each ofthe optical fiber cables in FIGS. 1 to 3.

FIG. 5 is a graph showing the relationship between MFD and spliced losswhen same-type optical fibers are spliced with each other.

FIGS. 6A and 6B are conceptual diagrams showing refractive indexprofiles of optical fibers according to an embodiment of the invention.

FIGS. 7A to 7F are conceptual diagrams showing index profiles of coresof optical fibers according to an embodiment of the invention.

FIG. 8 is a table showing structures and optical characteristics at awavelength of 1550 nm of optical fibers according to a comparativeexample and examples.

FIG. 9 is a table showing SSE when the optical fibers according toexamples are housed in optical fiber cables.

FIGS. 10A and 10B are graphs each showing the relationship between Aeffat a wavelength of 1.550 nm of an optical fiber and SSE of an opticalfiber cable.

FIGS. 11A and 11B are graphs each showing the relationship between Aeffat a wavelength of 1550 nm of an optical fiber and SSE of an opticalfiber cable.

FIG. 12 is a table showing slot sizes and other specifications ofoptical fiber cables according to examples.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Optical fiber cables according to an embodiment are described in detailwith reference to the accompanying figures. It is to be noted that theidentical reference sign is applied to the same elements in thedescription on the figures, and redundant description is omitted.

FIGS. 1 to 3 are cross-sectional views of optical fiber cables 10according to an embodiment of the invention. A structure of the opticalfiber cables 10 is properly selected depending on the arrangement placeand environment. Also, the number of optical fibers is determineddepending on the total required transmission capacity and the space ofthe conduit. In this case, FIG. 1, FIG. 2, and FIG. 3 show an opticalfiber cable including 40 optical fibers, 100 optical fibers, and 300optical fibers, respectively. Also, FIG. 4 is a cross-sectional viewshowing an example of a slot included in the optical fiber cable 10.

As shown in FIGS. 1 to 4, the optical fiber cable 10 is so-called ribbonslotted-core optical fiber cable. The optical fiber cable 10 includes aslotted core 1, a tension member 3, a sheath 4, a water seal tape 5, anda fiber ribbon 20.

The slotted core 1 is a resin rod made of, for example, a plastic havinga substantially cylindrical shape. A slot 2 is engraved in the slottedcore 1. The slot 2 is a groove for housing the fiber ribbon 20. Aplurality of the slots 2 are provided in the outer peripheral surface ofthe slotted core 1 along the axial direction of the optical fiber cable10. Five slots 2 are provided in each case of a cable having 40 opticalfibers and 100 optical fibers, and fifteen slots are provided in a caseof a cable having 300 optical fibers. The width of a bottom portion ofthe slot 2 is x1, the width of an upper portion of the slot 2 is x2, andthe depth of the slot 2 is y. In FIG. 4, x1 equals to x2; however, x1does not necessarily equal to x2.

The tension member 3 is arranged in a center portion of the slotted core1, and is integrally provided with the slotted core 1. The tensionmember 3 is made of fiber reinforced plastic (FRP). For example, FRP isformed by impregnating bound tensile-strength fibers with matrix resin,and the matrix resin is hardened by thermosetting.

The water seal tape 5 is wound around the outer peripheral surface ofthe slotted core 1 to cover the slot 2, and functions as a binding tape.The water seal tape 5 prevents the fiber ribbon 20 from protruding outfrom the slot 2. Also, the water seal rape 5 is formed of a waterabsorbing material, and prevents water from running in the longitudinaldirection of the optical fiber cable 10.

The sheath 4 is provided at an outermost peripheral portion of theoptical fiber cable 10. For example, the sheath 4 is formed of ahigh-strength plastic. To be specific, the sheath 4 may use ahigh-strength plastic, such as polyethylene, polyphenylene sulfide,polyether sulfon, polyether etherketone, or liquid crystal polymer. Thesheath 4 is formed by extruding a high-intensity plastic on the outerperiphery of the slotted core 1 covered with the water seal tape 5.

The fiber ribbon 20 includes a plurality of optical fibers 30 arrangedin parallel and integrated. In this embodiment, the fiber ribbon 20 isformed as a four fiber ribbon having integrated therein four opticalfibers 30. The fiber ribbons 20 are stacked by, for example, two in acase of a cable having 40 optical fibers, and five in each case of acable having 100 optical fibers and 300 optical fibers, and are housedin each slot 2.

In the optical fiber cable 10, a value A_(slot) obtained by dividing thesectional area (x1+x2)×y/2 by the number of fiber N_(slot) in a singleslot, that is, A_(slot)=(x1+x2)×y/(2×N_(slot)) is set in a range from0.12 to 0.16 mm²/core.

The definition of SSE and SSE of an optical fiber cable of related artare described next, and then SSE of the optical fiber cable 10 and anoptical fiber 30 according to this embodiment are described.

Definition of SSE

The limit of SE per one optical fiber can be obtained from the Shannonlimit, and is expressed by Eq. (2):SE=log₂(1+SNR),  (2)where SNR denotes a signal to noise ratio. The relationship between SNRand OSNR can be expressed by Eq. (3) as described in R. Essiambre et al,“Capacity Limits of Optical Fiber Networks,” Journal of LightwaveTechnology, Vol. 28, No. 4, pp. 662-701 (February, 2010):

$\begin{matrix}{{{SNR} = {\frac{2B_{ref}}{{pR}_{s}}{OSNR}}},} & (3)\end{matrix}$where p equals 1 in a case without polarization multiplex, and p equals2 in a case with polarization multiplex. Hereinafter, p equals 2. R_(s)denotes a symbol rate, and B_(ref) denotes a reference bandwidth ofOSNR, B_(ref) typically being 12.5 GHz (0.1 nm).

In an optical communication system using a digital coherent receiver,the maximum value of OSNR, OSNR_(max), is expressed by Eqs. (4) to (8)as described in M. Hirano et al, “Analytical OSNR Formulation Validatedwith 100G-WDM Experiments and Optimal Subsea Fiber Proposal,” OFC/NFOECTechnical Digest, OTu2B.6 (2013):

$\begin{matrix}{{{OSNR}_{\max} = {\left\{ {{\frac{4}{27F^{2}\eta} \cdot \gamma^{2}}L_{eff}} \middle| D \middle| {}_{- 1}{\cdot {\exp\left( {2\alpha\; L} \right)} \cdot A_{sp}^{2}} \right\}^{- \frac{1}{3}} \times N_{s}^{- 1}}},} & (4) \\{\mspace{79mu}{{N = {{NF} \cdot {hv} \cdot B_{ref}}},}} & (5) \\{\mspace{79mu}{{\eta = {\frac{8}{27} \cdot \frac{2v^{2}}{{CB}_{ch}^{3}} \cdot {{asinh}\left( \frac{\left. {\pi\; C} \middle| D \middle| {L_{eff}B_{t}^{2}} \right.}{4v^{2}} \right)} \cdot B_{ref}}},}} & (6) \\{\mspace{79mu}{{\gamma = {\frac{2\pi}{\lambda} \cdot \frac{n_{2}}{A_{eff}}}},\mspace{79mu}{and}}} & (7) \\{\mspace{79mu}{L_{eff} = {\frac{1 - {\exp\left( {{- \alpha}\; L} \right)}}{\alpha}.}}} & (8)\end{matrix}$

In the equations, γ denotes a nonlinear coefficient [1/W/km], D denotesa chromatic dispersion [ps/nm/km], a, denotes a transmission loss[1/km], n₂ denotes a nonlinear refractive index [m²/W], and Aeff denotesan effective area [μm²] of an optical fiber. In a case of standardsingle-mode fiber (standard SMF), at a wavelength of 1550 nm, thesevalues are set as follows: γ=1.2/W/km, D=17 ps/nm/km, α=0.047/km (0.185dB/km), n₂=2.35×10⁻²⁰ m²/W, and Aeff=80 μm². Also, L_(eff) denotes aneffective length [km], L denotes a span length (a repeater interval)[km], N_(s) denotes the number of spans, NF denotes a noise figure of arepeater (Erbium doped fiber amplifier, EDFA), h denotes a Planck'sconstant 6.63×10⁻³⁴ [Js], ν denotes a frequency [THz] of an opticalsignal, C denotes the speed of light 3×10⁸ [m/s], and B_(t) denotes awavelength division multiplexing (WDM) signal band [GHz]. A_(sp) denotesa splice loss between the optical fiber and the repeaters at both endsof the span, and can be obtained from MFD of the optical fiber andstandard SMF. If it is assumed that MFD of standard SMF at thewavelength of 1550 nm is 10.2 μm, A_(sp) of the optical fiber with MFDbeing W [μm] can be substantially expressed as [20×W/(W²+104)]².

In this case, for easier understanding, a case of transmitting a NyquistWDM signal in a transmission line composed of only a transmissionoptical fiber and a repeater is assumed; however, this case issubstantially established even in normal WDM transmission. With Eqs. (2)to (4), the limit of SE per one optical fiber is expressed by Eq. (9):

$\begin{matrix}{{SE} = {{\log_{2}\left( {1 + {\frac{B_{ref}}{R_{s}}{OSNR}_{\max}}} \right)} = {\log_{2}\left\lbrack {1 + {N_{s}^{- 1} \cdot {\left\{ {\left( {{NF} \cdot {hv} \cdot {\exp\left( {\alpha\; L} \right)} \cdot A_{sp}} \right)^{2} \cdot \left. \quad\left. {\gamma^{2}L_{eff}} \middle| D \middle| {}_{- 1}{\cdot \frac{4v^{2}}{C} \cdot {{asinh}\left( \frac{\left. {\pi\; C} \middle| D \middle| {L_{eff}B_{t}^{2}} \right.}{4v^{2}} \right)}} \right. \right\}^{- \frac{1}{3}}} \right\rbrack.}}} \right.}}} & (9)\end{matrix}$

Further, in a case of an optical fiber cable having a number N of fibersand a sectional area Ac [mm²], the limit of SSE is expressed by Eq.(10):

$\begin{matrix}{{SSE} = {\log_{2}\left\lbrack {1 + {{N_{s}^{- 1} \cdot \left\{ {\left( {{NF} \cdot {hv} \cdot {\exp\left( {\alpha\; L} \right)} \cdot A_{sp}} \right)^{2} \cdot \left. \quad\left. {\gamma^{2}L_{eff}} \middle| D \middle| {}_{- 1}{\cdot \frac{4v^{2}}{C} \cdot {{asinh}\left( \frac{\left. {\pi\; C} \middle| D \middle| {L_{eff}B_{t}^{2}} \right.}{4v^{2}} \right)}} \right. \right\}^{- \frac{1}{3}}} \right\rbrack} \times {\frac{N}{A_{C}}.}}} \right.}} & (10)\end{matrix}$If it is assumed that signal optical frequency ν=194 THz, noise figureNF=6 dB, span length L=100 km, number of spans N_(s)=15, and WDM signalband B_(t)=10 THz, and if it is assumed that pure silica core fibern₂=2.2×10⁻²⁰ [m²/W], SSE can be rewritten by Eq. (11):

$\begin{matrix}{{SSE} = {\log_{2}\left\lbrack {1 + {{65.9 \cdot \left\{ {{\exp\left( {200 \propto} \right)} \cdot \left( \frac{20 \times W}{W^{2} + 104} \right)^{- 4} \cdot \left. \quad\left. {A_{eff}^{- 2}L_{eff}} \middle| D \middle| {}_{- 1}{\cdot {{asinh}\left( {629 \cdot} \middle| D \middle| L_{eff} \right)}} \right. \right\}^{- \frac{1}{3}}} \right\rbrack} \times {\frac{N}{A_{C}}.}}} \right.}} & (11)\end{matrix}$SSE of Optical Fiber Cable of Related Art

A typical ribbon slotted-core optical fiber cable of related art has acable diameter of about 12 mm in the case where the cable includes 40fibers, about 17 mm in the case where the cable includes 100 fibers, andabout 23 mm in the case where the cable includes 300 fibers. Also, anoptical fiber housed in the optical fiber cable of related art usesstandard SMF.

A transmission loss of an optical fiber cable is equivalent to atransmission loss of an optical fiber if a bending loss or amicro-bending loss is not added when the optical fiber is housed in thecable. However, some optical fiber may have a transmission loss which isincreased by a bending loss or a micro-bending loss when the opticalfiber is housed in the cable. Alternatively, a bending loss or amicro-bending loss when an optical fiber is wound around asmall-diameter bobbin may be released and a transmission loss may bedecreased. A difference between a transmission loss after standard SMFis housed in a typical ribbon slotted-core cable of related art and atransmission loss of standard SMF was 0.00 dB/km in average and +0.018dB/km in maximum.

Then, if it is assumed that a value obtained by adding 0.02 dB/km to atransmission loss of the optical fiber (fiber transmission loss, fiberloss) serves as a transmission loss of a substantially standard opticalfiber cable (cable transmission loss Eq. (11) can be rewritten into Eq.(12):

$\begin{matrix}{{SSE} = {\log_{2}\left\lbrack {{1 + {{65.9 \cdot \left\{ {{\exp\left( {200 \cdot \left( \frac{\alpha_{dB} + 0.02}{4.343} \right)} \right)} \cdot \left( \frac{20 \times W}{W^{2} + 104} \right)^{- 4} \cdot \left. \quad\left. {A_{eff}^{- 2}L_{eff}} \middle| D \middle| {}_{- 1}{\cdot {{asinh}\left( {629 \cdot} \middle| D \middle| L_{eff} \right)}} \right. \right\}^{- \frac{1}{3}}} \right\rbrack} \times \frac{N}{A_{C}}}},} \right.}} & (12)\end{matrix}$where the fiber transmission loss is α_(dB) [dB/km]. With Eq. (12), SSEwhen standard SMF is housed in the typical ribbon slotted-core opticalfiber cable of related art is 1.5 b/s/Hz/mm² in the case where the cableincludes 40 fibers, 1.9 b/s/Hz/mm² in the case where the cable includes100 fibers, and 3.2 b/s/Hz/mm² in the case where the cable includes 300fibers.SSE of Optical Fiber Cable 10

In contrast, the optical fiber cable 10 has a cable diameter of about 11mm, 13 mm, 17 mm, or 19 mm in the case where the cable includes 40fibers, 100 fibers, 200 fibers, of 300 fibers respectively. The opticalfiber cable 10 satisfies Eq. (13):SSE≧0.008×N+1.7,  (13)where α_(dB) denotes a transmission loss [dB/km] of the optical fiber 30as described above. With Eq. (13), SSE can be increased by 30% or moreas compared with the ribbon slotted-core optical fiber cable of relatedart including the same number of standard SMFs. More preferably, Eq.(14) is satisfied:SSE≧0.009×N+2.0.  (14)Accordingly, SSE can be increased by 50% or more as compared with theribbon slotted-core optical fiber cable of related art including thesame number of standard SMFs. Further preferably, Eq. (15):SSE≧0.011×N+2.3  (15)is satisfied. Accordingly. SSE can be increased by 75% or more ascompared with the ribbon slotted-core optical fiber cable of related artincluding the same number of standard SMFs. It is to be noted that theleft sides of Eqs. (13), (14), and (15) are the same as the right sidesof Eq. (12).Optical Fiber 30

The optical fiber 30 housed in the optical fiber cable 10 according tothis embodiment is preferably a single core fiber for two reasons.

In a case of multi-core fiber, it is difficult to splice multi-corefibers of the same type with a low loss. When splicing is executed byusing a fusion splicer, for example, misalignment in a range from about0.2 to about 0.4 μm may be generated. In a multi-core fiber, sinceplural cores are housed in a single fiber, it is difficult to accuratelyalign the axes of all cores. In general, the length of a single opticalfiber cable arranged is in a range from about 1 to about 5 km. Hence, ina transmission line of a span length of 100 km, the number of same-typesplicing portions is 20 to 100 portions per span. Owing to this, thetotal span loss is largely increased by the splice loss. Also, themulti-core fiber is connected to devices at both ends of the span, adevice for branching each core is required, and the insertion lossthereof is added. Hence, the total loss is increased.

The optical fiber 30 preferably has the following features (1) to (5) atthe wavelength of 1550 nm.

(1) The transmission loss is preferably 0.19 dB/km or less or 0.18dB/kin or less. As the transmission loss is decreased. SSE can beincreased. The transmission loss is more preferably 0.17 dB/km or less.

(2) Aeff is preferably in a range from 100 to 125 μm² or 125 to 155 μm².As Aeff is increased, γ is decreased. Hence, SSE can be increased. Onthe other hand, if Aeff is excessively increased, a confinement effectfor propagation light in a core is decreased. Hence, the cabletransmission loss becomes larger than that of standard SMF due to thebending loss and micro-bending loss when housed in the optical fibercable 10.

(3) MFD is preferably in a range from 11.0 to 12.5 μm or 12.0 to 13.5μm. When same-type optical fibers 30 are spliced by using a fusionsplicer, for example, misalignment in a range from 0.2 to 0.4 μm may begenerated. FIG. 5 is a graph showing the relationship between asame-type splice loss and MFD. The horizontal axis indicates MFD at thewavelength of 1550 nm, and the vertical axis indicates the splice losswhen the same-type optical fibers 30 are spliced. As MFD is increased,the splice loss can be suppressed at a lower value even withmisalignment. In contrast, if MFD is excessively increased, the spliceloss with respect to standard SMF used as a pigtail of a repeater isincreased.

(4) A dispersion is preferably in a range from 19 to 22 ps/nm/km. As thedispersion is increased, SSE can be increased.

(5) A fiber cutoff wavelength is preferably in a range from 1400 to 1600nm. As the cutoff wavelength is increased, the bending loss can besuppressed at a lower value. The transmission loss after arrangement inthe cable can be maintained at a lower value. However, if the cutoffwavelength is excessively increased, single-mode transmission is nolonger executed.

FIGS. 6A and 6B are conceptual diagrams showing refractive indexprofiles of optical fibers 30 according to the embodiment. As shown inFIG. 6A, one example of the optical fiber 30 includes a core having arefractive index n1 and a diameter 2 a [μm], and a cladding having arefractive index n2. In this case, if it is assumed that a relativerefractive index difference of the core with respect to a refractiveindex no of pure silica Δ0 [%] is 100×(n1−n0)/n1 and a relativerefractive index difference of the core with respect to the cladding Δ1[%] is 100×(n1−n2)/n1, Δ1 is preferably in a range from 0.30% to 0.35%or 0.28% to 0.35% and 2 a is preferably in a range from 10 to 13 μm.Also, Δ0 is preferably in a range from −0.1% to +0.1%. It is effectivenot to substantially add a dopant to the core through which a major partof power of an optical signal propagates, in order to decrease the fibertransmission loss.

As shown in FIG. 6B, another example of the optical fiber 30 includes acore having a refractive index n1 and a diameter 2 a [μm], an innercladding having a refractive index n2 and a diameter 2 b [μm], and anouter cladding having a refractive index n3. The optical fiber has adepressed cladding type refractive index profile being n1>n3>n2. Sincethe optical fiber 30 has the depressed cladding type refractive indexprofile, the bending loss can be suppressed at a relatively low valueeven when Aeff is increased. Hence, this example of the optical fiber 30is preferable.

In this case, if it is assumed that a relative refractive indexdifference of the core with respect to the refractive index n0 of puresilica Δ0 [%] is 100×(n1−n0)/n1, a relative refractive index differenceof the core with respect to the inner cladding is Δ1 [%]=100×(n1−n2)/n1,and a relative refractive index difference of the outer cladding withrespect to the inner cladding is Δ2 [%]=100×(n3−n2)/n3, Δ1 is preferablyin a range from 0.30% to 0.35% or 0.28% to 0.35%, Δ2 is preferably in arange from 0.05% to 0.10%, 2 a is preferably in a range from 10 to 13μm, and 2 b is preferably in a range from 40 to 55 μm. Also, Δ0 ispreferably in a range from −0.1% to +0.1%. It is effective not tosubstantially add a dopant to the core through which a major part ofpower of an optical signal propagates, in order to decrease the fibertransmission loss.

FIGS. 7A to 7F are illustrations showing index profiles of cores ofoptical fibers according to this embodiment, The index profile of thecore of an optical fiber 30 can be modified in any one of variousshapes. In this case, it is assumed that the average value of refractiveindices of a core is n1.

As described above, the optical fiber cable 10 according to thisembodiment houses the plurality of optical fibers 30 and satisfies Eq.(1). Accordingly, SSE of the optical fiber cable 10 can be increased by30% as compared with the ribbon slotted-core fiber cable of related arthousing a standard single-mode fiber (standard SMF) when the numbers ofoptical fibers are the same. Also, since the transmission loss of theoptical fiber 30 at the wavelength of 1550 nm is 0.19 dB/km or less or0.18 dB/km or less, and Aeff is in the range from 100 to 125 μm² or 125to 155 μm², the transmission loss can be suppressed with an increasedSSE value.

Also, since MFD of the optical fiber at the wavelength of 1550 nm is inthe range from 11.0 to 12.5 μm or 12.0 to 13.5 μm, the splice lossbetween the same-type fibers can be suppressed at a low value even withmisalignment, and the splice loss with respect to standard SMF used asthe pigtail of the repeater can be suppressed at a low value. Also,since the dispersion of the optical fiber 30 at the wavelength of 1550nm is in the range from 19 to 22 ps/nm/km, SSE can be increased. Also,since the cutoff wavelength of the optical fiber 30 is in the range from1400 to 1600 nm, by increasing the cutoff wavelength of the opticalfiber cable within a range available for single-mode transmission, thebending loss can be suppressed at a low value and the transmission lossof the optical fiber in the cable can be maintained at a low value.

Also, in the one example of the optical fiber cable 10, the opticalfiber 30 includes the core and the cladding layer, the relativerefractive index difference Δ1 of the core with respect to the claddingis in the range from 0.30% to 0.35% or 0.28% to 0.35%, and the diameter2 a of the core is in the range from 10 to 13 μm or 12.0 to 13.5 μm.Accordingly, SSE of the optical fiber cable 10 can be increased.

Also, in the other example of the optical fiber cable 10, the opticalfiber 30 includes the core, the inner cladding, and the outer cladding.The refractive index n3 of the outer cladding is smaller than therefractive index n1 of the core and is larger than the refractive indexn2 of the inner cladding. The relative refractive index difference Δ2 ofthe outer cladding with respect to the inner cladding is in the rangefrom 0.05% to 0.10%, the relative refractive index difference Δ1 of thecore with respect to the inner cladding is in the range from 0.30% to0.35% or 0.28% to 0.35%, and the diameter 2 a of the core is in therange from 10 to 13 μm or 12.0 to 13.5 μm. Accordingly, even if Aeff isincreased, the bending loss can be suppressed at a relatively low value,and SSE of the optical fiber cable can be increased.

Also, since the relative refractive index difference Δ0 of the core withrespect to pure silica is in the range from −0.1% to +0.1%, and a dopantis not substantially added to the core through which the major part ofthe power of the optical signal propagates, the transmission loss of theoptical fiber 30 can be decreased.

Also, the optical fiber cable 10 is the ribbon slotted-core cable. Thevalue A_(slot) obtained by dividing the sectional area of the singleslot 2 by the number of optical fibers N_(slot) housed in the singleslot 2 is in the range from 0.12 to 0.16 mm²/core. If the sectional area(x1+x2)×y/2 of the slot 2 is decreased, the sectional area of the cablecan be decreased. This is advantageous to increase SSE of the opticalfiber cable 10. However, if the sectional area of the slot 2 isexcessively decreased, the clearance with respect to the fiber ribbon 20is decreased. Accordingly, the fiber ribbon 20 contacts the inner wallof the slot 2, and the bending loss and micro-bending loss are generatedby the stress of the optical fiber 30. Consequently, the loss isincreased more than the loss of the cable of related art, and SSE isdecreased. Particularly in a case of an optical fiber having Aeff beinglarger than that of standard SMF like the optical fiber 30 according tothis embodiment, the confinement for the propagation light in the coreis weakened, and the bending loss and micro-bending loss are likelygenerated. In this embodiment, since A_(slot) is determined within thepredetermined range, even if the above-described optical fiber 30 ishoused, the sectional area of the cable can be decreased while thedifference in transmission loss between the optical fiber 30 and theoptical fiber cable 10 is suppressed to be equivalent to a case in whichstandard SMF is housed in the typical ribbon slotted-core cable ofrelated art. Consequently, SSE of the optical fiber cable 10 can beincreased.

Also, the optical fiber cable 10 is the ribbon slotted-core cable, thevalue A_(slot) obtained by dividing the sectional area of the singleslot 2 by the number of optical fibers N_(slot) housed in the singleslot 2 is in the range from 0.12 to 0.16 mm²/core, the transmission lossof the optical fiber 30 at the wavelength of 15.50 nm is 0.19 dB/km orlower or 0.18 dB/km or lower, and Aeff is in the range from 100 to 125μm² or 125 to 155 μm². In this case, as described above, since A_(slot)is determined within the predetermined range, even if theabove-described optical fiber 30 is housed, the sectional area of thecable can be decreased while the difference in transmission loss betweenthe optical fiber 30 and the optical fiber cable 10 is suppressed to beequivalent to the case in which standard SMF is housed in the typicalribbon slotted-core cable of related art. Consequently, SSE of theoptical fiber cable 10 can be increased. Also, the transmission loss canbe suppressed with an increased SSE value.

Next, specific examples of the optical fiber according to thisembodiment are described in comparison with a comparative example. Anoptical fiber according to Example 1 was an optical fiber having arefractive index profile as shown in FIG. 6A. Optical fibers accordingto Examples 2 to 5 were optical fibers each having a depressed claddingtype refractive index profile as shown in FIG. 6B. An optical fiberaccording to the comparative example was standard SMF.

FIG. 8 is a table showing structures of optical fibers and opticalcharacteristics at the wavelength of 1550 nm according to thecomparative example and the examples. FIG. 9 is a table showing SSE whenthe optical fibers according to the examples were housed in opticalfiber cables. The difference between the cable transmission loss whenthe optical fiber according to any one of the examples was housed in theoptical fiber cable according to the embodiment of the invention and thetransmission loss shown in FIG. 8 was 0.02 dB/km or less even atmaximum, which is substantially equivalent to a case in which standardSMF is housed in the typical ribbon slotted-core cable of related art.Hence, similarly to the case in which standard SMF is housed in thetypical ribbon slotted-core cable of related art, SSE was obtained fromEq. (11) while the value obtained by adding 0.02 dB/km to the fibertransmission loss was treated as a substantially standard cabletransmission loss. In FIG. 9, “Y” was applied if obtained SSE satisfiedEqs. (13) to (15), and “N” was applied if obtained SSE did not satisfyEqs. (13) to (15).

FIGS. 10A, 10B, 11A, and 11B are graphs each showing the relationshipbetween Aeff of an optical fiber at the wavelength of 1550 nm and SSE ofan optical fiber cable. FIGS. 10A, 10B, 11A, and 11B shows the cases inwhich the optical fiber cable includes 40 fibers, 100 fibers 200 fibers,and 300 fibers, respectively. Each figure shows the relationship betweenAeff of the optical fiber at the wavelength of 1550 nm and SSE of theoptical fiber cable, for each of cases of fiber transmission lossesbeing 0.180, 0.170, and 0.160 dB/km. Also, each figure shows the valuesof the right sides in Eqs. (13) to (15).

In the range of Aeff from 70 to 125 μm², the difference in transmissionloss between the cable and the fiber is 0.02 dB/km or smaller even atmaximum, and similarly to the case in which standard SMF is housed inthe typical ribbon slotted-core cable of related art, the value obtainedby adding 0.02 dB/km to the fiber transmission loss is treated as asubstantially standard cable transmission loss. Referring to FIGS. 10A,10B, 11A, and 11B, as long as Aeff is 100 μm² or larger and the fibertransmission loss is 0.180 dB/km or lower, Eq. (13) can be satisfied.

FIG. 12 is a table showing slot sizes and other specifications ofoptical fiber cables according to examples. As shown in FIG. 12, withthe optical fiber cable according to any one of the examples, A_(slot)is set in the range from 0.12 to 0.16 mm²/core. Accordingly, even whenthe optical fiber is housed, the sectional area of the cable can bedecreased while the difference in transmission loss between the opticalfiber and the optical fiber cable is suppressed to be equivalent to thecase in which standard SMF is housed in the typical ribbon slotted-corecable of related art.

What is claimed is:
 1. An optical fiber cable having a sectional area ofAc [mm²] and housing a number N of optical fibers, wherein atransmission loss α_(dB) [dB/km], a mode field diameter W [μm], aneffective area Aeff [μm²], an effective length L_(eff) [km], and awavelength dispersion D [ps/nm/km] of each of the optical fibers at awavelength of 1550 nm satisfy Equation (1), $\begin{matrix}{\log_{2}\left\lbrack {{1 + {{65.9 \cdot \left\{ {{\exp\left( {200 \cdot \left( \frac{\alpha_{dB} + 0.02}{4.343} \right)} \right)} \cdot \left( \frac{20 \times W}{W^{2} + 104} \right)^{- 4} \cdot \left. \quad\left. {A_{eff}^{- 2}L_{eff}} \middle| D \middle| {}_{- 1}{\cdot {{asinh}\left( {629 \cdot} \middle| D \middle| L_{eff} \right)}} \right. \right\}^{- \frac{1}{3}}} \right\rbrack} \times \frac{N}{A_{C}}}} \geq {{0.008 \times N} + 1.7}} \right.} & (1)\end{matrix}$ wherein the transmission loss of the optical fiber at thewavelength of 1550 nm is 0.19 dB/km or less, and the effective area ofthe optical fiber is in a range from 125 to 155 μm².
 2. The opticalfiber cable according to claim 1, wherein the mode field diameter of theoptical fiber at the wavelength of 1550 nm is in a range from 12.0 to13.5 μm.
 3. The optical fiber cable according to claim 1, wherein thewavelength dispersion of the optical fiber at the wavelength of 1550 nmis in a range from 19 to 22 ps/nm/km.
 4. The optical fiber cableaccording to claim 1, wherein the optical fiber has a cutoff wavelengthin a range from 1400 to 1600 nm.
 5. The optical fiber cable according toclaim 1, wherein the optical fiber comprises: a core having a diameterin a range from 12 to 15 μm; and a cladding surrounding the core andhaving a refractive index that is smaller than a refractive index of thecore, wherein a relative refractive index difference of the core withrespect to the cladding is in a range from 0.28% to 0.35%.
 6. Theoptical fiber cable according to claim 5, wherein a relative refractiveindex difference of the core with respect to pure silica is in a rangefrom −0.1% to +0.1%.
 7. The optical fiber cable according to claim 1,wherein the optical fiber comprises: a core having a diameter in a rangefrom 12 to 15 μm; an inner cladding surrounding the core and having arefractive index that is smaller than a refractive index of the core;and an outer cladding having a refractive index that is smaller than arefractive index of the core and larger than a refractive index of theinner cladding, wherein a relative refractive index difference of theouter cladding with respect to the inner cladding is in a range from0.05% to 0.10%, and a relative refractive index difference of the corewith respect to the inner cladding is in a range from 0.28% to 0.35%. 8.The optical fiber cable according to claim 7, wherein a relativerefractive index difference of the core with respect to pure silica isin a range from −0.1% to +0.1%.
 9. The optical fiber cable according toclaim 1, wherein the optical fiber cable is a ribbon slotted-core cable,and wherein a value obtained by dividing a sectional area of a singleslot by a number of cores of the optical fibers housed in the singleslot is in a range from 0.12 to 0.16 mm²/core.